When a junction is created between two dissimilar metals, a voltage is produced that depends both on the composition of the two metals and the temperature of the junction. Such a junction is known as a "thermocouple junction". The voltage produced by the thermocouple junction can be measured at different temperatures and recorded. Subsequently, an unknown temperature can be determined by putting the thermocouple in its environment, measuring the voltage it produces and comparing it with the stored voltage versus temperature data.
In practice, since the voltage of the thermocouple junction must be measured, the metals forming the junction have to be part of a circuit, and therefore there must be at least one other junction of dissimilar metals somewhere in that circuit. Thus, temperature measurements using thermocouples are performed by a circuit having both a measurement junction, and a reference, or "cold", junction. The voltage measured then represents the difference between the voltages produced by the measurement junction and the reference junction. If the temperature of the reference junction and the voltage-versus-temperature characteristic of the two metals used in the thermocouple junctions are both known, then the temperature at the measurement junction can be accurately established. The accuracy of a temperature measurement made with a thermocouple junction is thus limited by the accuracy of the temperature measurement of the temperature of the reference junction.
An alternative to measuring the voltage at the reference junction is to use some means for accurately establishing and maintaining that temperature at a predetermined level. For example, the known temperature of phase change, such as that created by mixture of water and ice. Hence, the name "cold junction". Obtaining extreme accuracy by this method also requires measurement of the ambient pressure and compensation for its deviation from the standard value. Several other methods and variations on methods are outlined in the following discussions of patents.
U.S. Pat. No. 3,650,154 to Arnett et al. for "Thermocouple Input Temperature Sensing Circuitry", hereby incorporated by reference, described circuitry that compares the thermocouple output voltage with a reference voltage and amplifies the difference. The reference voltage varies with ambient temperature in order to null out the effects of temperature change on the cold junction. The cold junction is disposed within the same region as the temperature indicating circuitry, so that differences between their temperatures are minimized.
U.S. Pat. No. 5,088,835 to Shigezawa et al. for "Reusable Probe Connector Apparatus", hereby incorporated by reference, discloses a phone plug apparatus housing in close proximity with both a cold junction and a thermistor. The thermistor output is chosen so as to compensate as closely as possible for the voltage generated by the cold junction, while their physical proximity in the plug are intended to assure that the cold junction and thermistor are actually operating at the same temperature. Electrically, the thermistor, cold junction and hot junction are all connected in series. The thermistor produces a change in voltage with respect to temperature that is opposite in polarity from the change in voltage with respect to temperature produced by the cold junction. With those two voltages canceling, the net voltage measured by circuitry in the instrument is interpreted as exclusively that coming from the hot junction.
U.S. Pat. No. 4,482,261 to Dewey et al. for "Method for Simultaneous Reference Junction Compensation of a Plurality of Thermocouples", hereby incorporated by reference, describes how multiple reference junctions are held to the same temperature by a metallic isothermal block. The temperature of that block is then monitored by an integrated circuit temperature transducer, the output of which is converted to a temperature by a microprocessor using either formula or lookup table based calculations.
U.S. Pat. No. 4,718,777 to Mydynski et al. for "Isothermal Block for Temperature Measurement System Using a Thermocouple", hereby incorporated by reference, disclosed a method for compensating for the temperature of the reference junction by maintaining it and a temperature sensor in an isothermal block. The temperature sensor tracks the temperature of the reference junction so that its effect could be accurately accounted for in calculating the temperature of the measurement junction. The focus of this patent is on the isothermal block, its high thermal conductivity, and its ability to mount to a printed circuit board.
The problem of determining and/or compensating for the temperature of the reference junction in a thermocouple-based temperature measurement system is made more difficult if the measurement system is mobile, such as is particularly the case if it is handheld. When the handheld instrument is moved to a new environment, various parts of the instrument will take different amounts of time to reach thermal equilibrium with the new ambient temperature.
U.S. Pat. No. 5,167,519 to Jones et al. for "Temperature Compensating Universal Connector", hereby incorporated by reference, describes a connector assembly having thermal and electrical features that make it very effective for connecting a mobile instrument to thermocouple sensors prepositioned in environments of varying temperature and through plugs and wires of different sizes and styles. The isothermal mass is minimized and the means for effective thermal coupling are maximized in order to optimize the transient response of the connector system as it is connected to thermal probes in different temperature environments.
Handheld instruments that are used exclusively for temperature measurement can have (and have had) an internal isothermal block. By thermally isolating this isothermal block from the rest of the instrument as much as possible, a temperature sensor located within this isothermal block can accurately track the temperature of a reference junction that is also embedded in the same isothermal block.
Multi-function handheld meters, particularly those that are primarily designed for making electrical measurements, such as voltage, current, resistance, and frequency, have size, weight, cost and safety concerns that make such a dedicated integral isothermal block more problematical and less desirable.
Nonetheless, if these multimeters are to make accurate temperature measurements, they must have some way to provide reference junction temperature measurement or compensation that is accurate and stable, as the handheld instrument is moved about to make measurements in environments having different ambient temperatures.